The present invention relates to a method and device for determining the density of a stream of fibrous material on a cigarette manufacturing machine.
In the following description, reference is made purely by way of example to a cigarette manufacturing machine and to determination of the tobacco density of a continuous cigarette rod.
As is known, on cigarette manufacturing machines, a suction conveyor belt draws the tobacco from a tank and deposits it on to a continuous strip of paper; the longitudinal edges of the paper strip are then folded one on top of the other about the tobacco; and the continuous cigarette rod so formed is fed to a cutting station where it is cut into single or double cigarettes.
The tobacco is normally supplied in such a manner as to be distributed unevenly inside the cigarette, and more specifically in such a manner as to be denser at the two ends than in the center, to prevent tobacco fallout and detachment of the filter from the cigarette, and at the same time ensure correct ventilation of the intermediate portion of the cigarette. This is achieved by supplying a greater quantity of tobacco at the ends of the cigarette as compared with the center, for which purpose, a rotary shaving device is provided along the path of the tobacco on the conveyor, for shaving it into the contour corresponding to the required density. The shaving device is both height adjustable for controlling the mean quantity of tobacco in each cigarette (mean density or weight), and time adjustable for obtaining a maximum quantity of tobacco at the point at which the continuous cigarette rod is cut (adjacent ends of two cigarettes); which adjustment is made according to the discrepancy between the desired distribution of the tobacco and the actual distribution determined on the cigarette rod upstream from the cutting station.
Various solutions currently exist for determining the actual distribution of the tobacco, most of which feature a beta-ray sensor comprising a radioactive source and a beta ray detector located on either side of the cigarette rod, along the path of the rod between the forming and cutting stations. The radioactive source typically comprises a strontium (Sr90) pellet, and is housed inside a shielded container with a hole facing the cigarette rod; and the detector comprises an ionization chamber and an electrometer for measuring the energy of the incoming radiation. On the basis of fluctuations in the incoming radiation, an electronic system connected to the detector determines the variation in the density of the tobacco and controls the shaving knife accordingly.
Though precise and reliable, the above solution creates numerous problems, mainly due to the use of harmful radiation which, on the one hand, requires special care and procedures on the part of the operators, and, on the other, poses problems for disposing of the depleted pellets. All these problems are further compounded by the energy of the emitted radiation being correlated to the traveling speed of the cigarette rod, and by the current tendency to produce increasingly fast-operating machines therefore requiring greater amounts of energy. As a result, alternative solutions have been devised featuring different types of sensors, the efficiency of which, however, is impaired by the sensors being sensitive to different parameters such as the humidity, colour and more or less fibrous structure of the tobacco.